1. Field of the Invention
The present invention relates to a developing apparatus to be used in an image forming apparatus such as a copier and a laser beam printer, which utilizes an electrostatic recording process or an electrophotographic printing process in which an electrostatic image formed on an image bearing member is developed with developer including toner and carrier.
2. Description of the Related Art
Conventionally, an image forming apparatus using an electrophotographic printing process develops, with use of a developing apparatus, an electrostatic latent image formed on a photosensitive member as an image bearing member into a visible toner image with toner included in developer.
The most typical developing apparatus includes a developing container configured to contain developer, a conveyance member configured to covey the developer in the developing container while agitating and mixing the developer, a developer carrying member configured to carry and convey the developer up to a portion opposite to a photosensitive member, and a layer thickness regulating member configured to regulate an amount of the developer on the developer carrying member.
In this context, a developing apparatus which uses a two-component developer including a non-magnetic toner and a magnetic carrier will be described. The developer contained in the developing container is agitated and mixed by a developing screw serving as a conveying member in the developing container. The developer is electrically charged by the triboelectrical charging during the agitation and mixing. The developer thus electrically charged is carried by a developing sleeve as a developer carrying member, which includes therein a magnet having a plurality of magnetic poles and serving as a magnetic-field generating unit, with the aid of a magnetic force of the magnet mainly. This developing sleeve is rotatably arranged at a position opposing the photosensitive member. By rotation of the developing sleeve, the developer is conveyed to a developing region as the opposing portion with respect to the photosensitive member, and subjected to development. In the developing region, a developing bias applied to the developing sleeve causes the toner included in the developer to be transferred onto the electrostatic latent image formed on a surface of the photosensitive member, and a toner image corresponding to the electrostatic latent image is formed on the surface of the photosensitive member. In general, in many of such developing apparatus, a regulating blade as the layer thickness regulating member is arranged to oppose an outer peripheral surface of the developing sleeve with a predetermined gap between them. There have been proposed various regulating blades, such as a magnetic plate regulating blade, a non-magnetic plate regulating blade, a combination thereof, and an elastic regulating blade, and such regulating blades have been practically used. When the developer is conveyed to the developing region, the amount of the developer carried by the developing sleeve is regulated at a time of passing through the gap between the developing sleeve and the regulating blade. In this way, the developer is controlled to be supplied by a stable amount. Normally, one of the magnetic poles of the magnet (referred to as a cutting pole) is opposed to an opposing portion of the regulating blade so that the developer amount is regulated while a developer pool is formed. In such a configuration, a certain amount of the developer can be constantly secured in an immediately upstream portion with respect to the regulating blade, and hence the developer can be stably supplied to the developing sleeve.
As a developing sleeve configured to efficiently convey the two-component developer, there has been conventionally known a developer carrying member having a surface roughened by a sand-blasting process. The roughened surface of this developing sleeve has higher frictional resistance, and hence more developer can be drawn-up and conveyed. However, along with abrasion due to rubbing against the developer, the surface is gradually smoothened, which causes reduction in developer conveying amount with the passage of time. Further, magnetic carrier particles contained in the developer are rubbed against fine projections and recesses on the surface, and hence the magnetic carrier particles themselves are also abraded. As a result, the reduction in developer conveying amount becomes more serious with the passage of time. In addition, graininess appears in the visible image after development, and uneven image density in halftone portions thereof becomes conspicuous. For those reasons, it is significantly difficult to maintain high image quality over a long period.
In view of the circumstances, as a developer carrying member configured to suppress the graininess and the uneven image density, there has been known a developer carrying member having a surface provided with a plurality of V-grooves. With the developer carrying member of this type, developer particles can be captured by the plurality of grooves formed in the surface, and efficiently conveyed. In addition, even after the surface is abraded, the frictional resistance does not significantly vary, and the abrasion of the magnetic carrier particles is reduced. Thus, deterioration in image quality with the passage of time can be effectively suppressed.
However, when the above-mentioned conventional developing sleeve provided with the V-grooves was used over a long period, the developer was compressed and subjected to shear by the back of the regulating blade. As a result, toner particles clog the recessed portions of the V-grooves, which may cause deterioration in developer conveying property and occurrence of an uneven pitch image due to a groove period. Specifically, the developer compressed on the backside of the regulating blade is conveyed partially in a flow toward the surface of the developing sleeve while being guided by a surface of the regulating blade, and hence the toner particles clog the V-grooves. In particular, as for the two-component developer including a toner and a magnetic carrier, particles of the developer are attracted to the surface of the developing sleeve by the magnet surrounded by the developing sleeve, to thereby hold the developer. In this state, as described above, the developer is compressed particularly on the backside of the regulating blade, and hence the toner is subjected to shear. As a result, particles of the toner gradually clog the recessed portions of the V-grooves.